CSL has developed a computer system, now in clinical operation in the Radiation Oncology Branch, NCI, to use the detailed contour and density information available from computer assisted tomography to improve radiation treatment planning. This sytem for external beam treatment planning is based on a generalized 3-D dose field model that covers photon, electron, and neutron beams. The computer program and most of its clinical implementation has been completed for the photon and electron fields available from the local 6 MV and 12 MV linear accelerators. The current capabilities include interactive simulation of most irradiation techniques, including the effect of most beam modifying devices. The system enables the display of dose distributions computed in several transverse contours and overlaid on corresponding CT scans.